1. Field of the Invention
The present invention relates to circuitry for a high voltage integrated circuit (IC), and, more specifically, to a differential amplifier circuit that can amplify a differential signal in the hundreds of millivolts near the high voltage power supply, minimize inherent temperature offset drift, and generate a high side current reference in a current sense IC.
2. Description of the Related Art
The circuitry of a high voltage current sense IC, such as the IR2171 current sense IC sold by International Rectifier Corporation of El Segundo, Calif. are disclosed in U.S. patent application Ser. No. 09/266,822 filed Mar. 12, 1999, the entire disclosure of which is incorporated herein by reference.
The IR2171 provides a circuit for transferring static or time variable analog information without electrical isolation from a first (source) reference potential to a second (destination) reference potential.
More specifically, the IR2171 circuit recovers an input signal at a first potential which is offset by a common mode displacement from a second potential. The circuit in its most basic form includes: (1) circuitry for converting the input signal at the first potential to a pulse width modulated signal; and (2) circuitry for level shifting the pulse width modulated signal from the first potential to the second potential. The IR2171 advantageously can be used in a motor controller for transferring information relating to current flow through a high side resistor from a high voltage potential to a lower level potential for conditioning and processing the information.
Desirable features for a high voltage current sense IC, such as the IR2171, include a differential amplifier that can amplify a differential signal in the hundreds of millivolts near the power supply, minimize inherent temperature offset drift, and generate a high side current reference.
FIG. 1 shows a typical prior art differential amplifier circuit 2. Circuit 2 includes a differential amplifier 4 biased by four matching resistors 6, 8, 10, and 12. The advantage of the prior art differential amplifier circuit, such as the circuit shown in FIG. 1, is that Vin can be amplified with a CMIV (common mode input voltage) from VSS⊃−1V to close to VDD.
The disadvantages of the circuit of FIG. 1 include the following: (1) four matching resistors 6, 8, 10, and 12 are required; (2) the four resistors occupy valuable space on the die; (3) the CMRR (common mode rejection ratio) depends on how closely the resistors are matched, which is typically not better than 1% in an IC implementation; (4) the offset also depends on how closely the resistors are matched; and (5) Vref, which needs to supply current in 10 and 12, should have a low impedance output; in practice, Vref is implemented as a voltage reference and follower op-amp 14, as shown in FIG. 2.
FIG. 3 shows another prior art differential amplifier circuit 16. Circuit 16 includes differential amplifier 18, resistors 20 and 22, and provided with a Vref 24. The advantages of circuit 16 of FIG. 3 are as follows: (1) only two resistors 20 and 22 need be matched, which is much easier and results in a CMMR of better than 0.5%; (2) the CMRR is improved with respect to circuit 2 of FIG. 1, and the offset is better, both due to the reduced number of resistors required (resulting in improved resistor matching); and (3) Vref is a high impedance load, and therefore be a simple resistor divider; since there is no need for the buffer of FIG. 2, the circuit occupies a small area. The disadvantage of circuit 16 of FIG. 3 is that it can only have a CMIV of VSS, since Vin is referenced to VSS.
A further shortcoming of the prior art is that conventional op amps have an input offset voltage which is temperature sensitive. It would be desirable to provide an op amp circuit in which the input offset voltage is constant and independent of changes in temperature.
High voltage current sense ICs require a high side current reference. Referring to FIG. 4, this is typically accomplished by providing an NPN transistor 22 which has its collector tied to a VB supply. As described more fully below, by regulating the voltage of emitter resistor 24 of the NPN transistor and repositioning resistor 24, a ΔVbe/R current reference can be implemented on the high-side. Very good tolerance (±10%) and power supply rejection ratio has been implemented using this approach.